Tris(1,10-phenanthroline-κ2 N,N′)nickel(II) dinitrate tetrahydrate

In the title complex, [Ni(C12H8N2)3](NO3)2·4H2O, the NiII ion is octahedrally coordinated by three bidentate 1,10-phenanthroline ligands, each forming a five-membered chelate ring. In the crystal, O—H⋯O and C—H⋯O hydrogen bonds are present between the complex cations, nitrate anions and water molecules. O—H⋯O hydrogen bonds between the uncoordinated water molecules lead to the formation of a four-membered ring water cluster, with a planar configuration. There were an additional five grossly disordered water molecules in the asymmetric unit, which were removed by the subroutine SQUEEZE; these were were excluded in the calculation of the molecular weight, etc. π–π stacking interactions between the aromatic rings are also observed [centroid–centroid distances = 3.697 (2), 3.728 (2) and 3.761 (2) Å].

In the title complex, [Ni(C 12 H 8 N 2 ) 3 ](NO 3 ) 2 Á4H 2 O, the Ni II ion is octahedrally coordinated by three bidentate 1,10-phenanthroline ligands, each forming a five-membered chelate ring. In the crystal, O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds are present between the complex cations, nitrate anions and water molecules. O-HÁ Á ÁO hydrogen bonds between the uncoordinated water molecules lead to the formation of a fourmembered ring water cluster, with a planar configuration. There were an additional five grossly disordered water molecules in the asymmetric unit, which were removed by the subroutine SQUEEZE; these were were excluded in the calculation of the molecular weight, etc.stacking interactions between the aromatic rings are also observed [centroid-centroid distances = 3.697 (2), 3.728 (2) and 3.761 (2) Å ].

Comment
Metal complexes with polypyridine-containing ligands are of great interest to researchers, and a number of reports have been devoted to this theme. Among polypyridine-containing ligands, 1,10-phenanthroline (1,10-phen) is a very important pyridine derivative, which has attracted much interest in coordination chemistry. Transition metal complexes coordinated with 1,10-phen show excellent photoelectrical capability. The application of a Ni-phenanthroline complex for detection of nucleic acid has also been reported (Qiua et al., 2011). Extensive investigations of small and medium size water cluster structures (H 2 O) n (n = 2-100) have been reported in recent years (Rodríguez-Cuamatzi et al., 2004). Among the water clusters, the cyclic (H 2 O) 4 is very interesting as it is a simple two-structure model for liquid water. Recently we have reported a tetrameric water cluster ring in the crystal structure of a new proton transfer system derived from pyridine-2,6-dicarboxylic acid and 2-amino-4-methylpyridine (Sharif et al., 2010). Here, we present the preparation and the crystal structure of the title compound and the formation of a tetrameric water cluster with a planar configuration.

Refinement
The asymmetric unit of the title compound contains nine water molecules, five of which were disordered and were therefore removed by the command SQUEEZE in PLATON (Spek, 2009). They have been excluded in the calculation of molecular weight, crystal density and absorption coefficient. Although the H atoms were located from difference Fourier maps, they were included at geometrically idealized positions with O-H = 0.82 and C-H = 0.95 Å and with U iso (H) = 1.2U eq (O,C). Fig. 1. Molecular structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level.

Special details
Experimental. IR Spectra IR spectra were recorded using FTIR Spectra Bruker Tensor 27 spectrometer (KBr pellets, 4000-400 cm -1 ). TGA-DTA measurements were performed at heating rate of 10 K min -1 in the temperature range of 298-1273 K, under nitrogen flow of 20 ml min -1 on instrument Perkin Elmer Pyris Diamond Thermogravimetric/Differential Thermal Analyzer. Elemental analyses were performed using a Costech ECS 4010 CHNS analyzer.
The FTIR spectrum of the crystals shows broad strong bands at the region 3000-3500 cm -1 . In the spectra of the phenanthroline complexes strong bands are observed in three frequency regions, between 700 -900 cm -1 , 1125 -1250 cm -1 , and 1400 -1650 cm -1 (Schilt & Taylor 1959). In the title complex, these bonds were observed in the regions 721-869 cm -1 , 1138-1225 cm -1 and 1429-1573 in the IR spectra.